The Rational Control of Precursor Concentration in Perovskite Light-Emitting Diodes

Abstract

Perovskite light-emitting diodes (PeLEDs) have attracted tremendous attention due to their ideal optoelectronic properties, such as high color purity, high fluorescence quantum yield, and tunable light color. The perovskite layer plays a decisive role in the performance of PeLEDs and the solvent engineering of the perovskite layer is the key technological breakthrough in preparing high quality films. In this study, we have proposed the strategy of adding different amounts of solvents to the perovskite precursor solution to optimize the morphology of perovskite films and device performance. As a result, with the decreasing concentration of perovskite precursor solution, the perovskite film morphology is smoother and more favorable for carrier injection and combing, which induces an enhanced external quantum efficiency. The maximum luminance of PeLEDs was increased from 1667 cd/m² to 9857 cd/m² and the maximum current efficiency was increased from 6.7 cd/A to 19 cd/A. This work provides a trend to achieve improved film morphology and device performance for perovskite optoelectronic devices.

1. Introduction

Perovskite light-emitting diodes (PeLEDs) have attracted tremendous attention due to their ideal optoelectronic properties, such as high color purity, high fluorescence quantum yield, and tunable light color [1,2,3]. During the past years, the performance of PeLEDs increased dramatically [4,5,6,7], which was mainly induced by the improved perovskite layer quality [8,9,10]. Several strategies have been utilized to achieve the high quality perovskite thin films, for example, through interfacial modification to improve the surface properties [11,12,13], through different solvents or mixed solvents to optimize the crystallinity of the films [14,15,16], or the use of additives to passivate the film defects [17,18]. In 2018, Gao et al., obtained thin films with a smooth surface, high crystallinity, and low defect density by using dimethyl sulfoxide (DMSO) and MACl synergistically, and adding these two additives to the perovskite precursor solution [19]. Zhao et al., introduced appropriate organic additives, to improve the optoelectronic properties of perovskite to prepare efficient and flexible PeLEDs [20]. In 2019, Meng et al., improved the crystallinity and carrier transport of perovskite films by optimizing both the dissolving solvent and the anti-solvent with DMF [13]. In 2021, Yang et al., developed a quasi-dimensional perovskite film formation process by adding a small amount of methane sulfonate, which effectively reduced the surface defect state of the films [21].

In addition, solvent engineering is one of the key technologies in preparing the high quality perovskite films. Compared with other solvent methods, perovskite films prepared with DMSO have a uniform, dense morphology, high crystallinity, high carrier mobility, and high conductivity, which results in superior device performance [22,23]. However, the effect of perovskite precursor concentration on the DMSO solution was less explored [13,22]. In this paper, we have investigated the effect of the perovskite precursor concentration on film crystallization and device performance. We have characterized the morphology of perovskite films with different concentrations. Meanwhile, the PeLEDs were fabricated and characterized with them as an active layer.

2. Materials and Methods

The detailed film preparation and characterization are in the supporting information.

The filtered poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) was spin-coated onto the ITO (indium tin oxide) glass substrate at a speed of 5000 rpm for 40 s and then annealed at 150 °C for 30 min. A total of 2 mg/mL poly(9-vinylcarbazole) (PVK) film was subsequently spin-coated at speed of 5000 rpm for 40 s. The perovskite PEA₂PbBr₄(CsPbBr₃)₄ precursor solution was prepared by dissolving stoichiometric precursor powder in different DMSO solutions, which were 0.5 M, 0.33 M, 0.25 M, and 0.17 M. They were spin-coated onto ITO/PEDOT:PSS/PVK films at a rate of 3000 rpm and 40 s in a nitrogen glove box. The above prepared samples were placed on the mask plate with the perovskite layer facing downwards in a vacuum evaporator to deposit the TPBi (45 nm)/LiF (3 nm)/Al electrode (100 nm).

3. Results

The spin-coated thin films formed by the various perovskite precursor concentrations were irradiated under a room lamp and UV lamp, respectively. The corresponding optical pictures are shown in Figure 1a. Under the indoor light irradiation, the optical images showed that these films were yellow-green, and with the decreased perovskite precursor concentrations, the films are relatively flattering and the perovskite grains are denser. The thickness of the perovskite films gradually decreases. The film color becomes less bright. This was due to the gradual thinning of the films, as indicated in Figure S1. Under the UV lamp irradiation, the optical pictures showed a bright, uniform green color, and it was observed that the films with a concentration of 0.50 M were relatively bright.

Photoluminescence (PL) characterization was performed on the films formed by different concentrations of perovskite precursor solutions, and the four PL spectral curves are shown in Figure 1c. The PL intensity of the film with the concentration of 0.5 mmol/mL (M) was the highest. This matches the results obtained from the UV light irradiation of the real object. With the decrease in concentration of the perovskite precursor solution, a slight red shift of the PL spectral peaks can be seen, in Figure 1d, which is possibly due to local variations of the perovskite grain sizes and defect conditions [24,25,26].

In order to investigate the effect of perovskite precursor concentrations on the electroluminescence (EL) characteristics of PeLEDs, we have prepared the devices with the structure of ITO/PEDOT:PSS/PVK/Perovskite/TPBi/LiF/Al, as shown in Figure 2a. The energy band structures of all functional layers are shown in Figure 2b. The relationship between the driving voltage and external quantum efficiency (EQE) shows that, with the decreased in the perovskite precursor concentration, the driving voltage of the maximum EQE gradually decreased, as shown in Figure 2c. This means that, as the perovskite precursor concentration decreases, the perovskite film’s morphology is smoother, and the defect state in the perovskite layer is suppressed and the non-radiative complexation is reduced; therefore, it is more favorable for the preparation of high efficiency PeLEDs. The current density of the sample with the a concentration of 0.17 M within 3 to 9 V is relatively lower than that of the sample with the concentration of 0.25 M. Moreover, the short-circuit current density of the sample with the concentration of 0.5 M and the sample with the concentration of 0.33 M, is relatively too low within this interval of voltage, which can lead to the lower efficiency of the device. The luminance–voltage curves were found to increase and then decrease for all four devices. When the applied voltage is within 3 to 8 V, the sample of concentration of 0.25 M has a larger luminance than the sample of concentration of 0.17 M, but the luminance of the sample of concentration of 0.17 M is significantly larger after the voltage over 8 V, as shown in the luminance–voltage curve, in Figure 2e, which indicates that the sample of concentration of 0.17 M withstands a larger voltage and the device is more stable. Moreover, the sample with the concentration of 0.17 M has a larger maximum luminance than the sample with a concentration of 0.25 M. In order to visualize the comparison results of the maximum EL curves of the PeLEDs prepared with four different concentrations of perovskite precursor solutions, the maximum EL curves were plotted, as shown in Figure 2f. It was found that the maximum EL intensity of the PeLEDs prepared with the decreased concentration of the perovskite precursor solution, gradually increased. The normalized EL curves showed that the luminescence peaks were red-shifted with the decreased perovskite precursor concentrations, which can be caused by the reduced non-radiative recombination and the eliminated trap states of the perovskite film [24,25]. Moreover, the differences in the EL peaks between 0.33 M and 0.25 M were not significant, as is shown in the inset of Figure 2f.

To further investigate the EL characteristics of the PeLEDs made with different perovskite precursor concentrations, the EL intensity curves were obtained by increasing the voltage from 3 to 12 V, as shown in Figure 3. When the driving voltage was increased from 3 V to 12 V, the EL intensity of the PeLEDs prepared with four different concentrations of the perovskite precursor solutions increased to the maximum, and then decreased; however, there was no peak shift in the EL spectra, which means green luminescence with high color purity was obtained. With the decrease in the concentrations of the perovskite precursor solutions, a relatively high EQE of the device can be obtained at a relatively low driving voltage, with the maximum luminance increasing from 1667 cd/m² to 9857 cd/m² and the current efficiency increasing from 6.7 cd/A to 19 cd/A. The detailed performance parameters are summarized in

Table 1. Device performance of the PeLEDs made from different concentrations of the perovskite precursor solutions.

Concentration (mmol/mL)	Thickness (nm)	EL1 peak position (nm)	EQE1max Driving voltage (V)	EQEmax (%)	L1max (cd/m2)	CE1max (cd/A)
0.50	180	514	7	2.09	1667	6.7
0.33	120	518	6	2.12	3143	7.5
0.25	60	518	5	3.07	4400	11.1
0.17	40	520	5	5.17	9857	19.1

¹ EL: electroluminescence; EQE: external quantum efficiency; L: luminance; and CE: current efficiency.

.

4. Conclusions

In this study, we have investigated the effect of perovskite precursor concentrations on the film morphology and device performance in PeLEDs. It was found that, with the decreased precursor concentrations, the perovskite film became smoother, which induced favorable carrier recombinations in the perovskite layer. As a result, with the decreased precursor concentrations, the device performance of the PeLEDs was increased, the maximum luminance was increased from 1667 cd/m² to 9857 cd/m², and the current efficiency was increased from 6.7 cd/A to 19 cd/A. Due to the decreased precursor concentrations, the perovskite film morphology was smoother, the defect state in the perovskite layer was suppressed, and the non-radiative complexation was reduced, ensuring a more efficient complexation of holes and electrons. This work provides a trend to achieve improved film morphology and device performance for perovskite optoelectronic devices.